Plain miter joints are extremely difficult to assemble permanently in registered alignment unless a holding fixture is used. Such fixtures are impractical for use other than "in-shop," particularly for long boards for hollow beams, boxes, and columns. Many modifications have been made to the miter surfaces to facilitate such miter joint assembly, as seen in U.S. Pat. Nos. 909,080 and 1,089,360, on page 9 of a current Anthe Machine Works catalog, and on page 1447 of Websters Third New International Dictionary of the English Language Unabridged, Copyright 1966. However, none of these prior art arrangements have the advantages of strength, non-locking assembly, ease of assembly, and diversity of assembly offered by the present invention.